WO 0243892 A2 provides a tool for forming slots in metal sheets. The tool comprises an upper tool part with a rectangular punch and a lower tool part with a opening adapted to the cross section of the punch. The punch has a cutting edge as a processing device. The cutting edge is inclined on the longitudinal sides of the punch relative to the plane of the metal sheet and on a transverse side of the punch perpendicular to the longitudinal sides. Two counter cutting edges are provided as counter devices on the opening which are arranged respectively on a transverse side and on the longitudinal sides of the opening. At the beginning of the slotting process a strip is cut out which is still joined onto the metal sheet on one side, in that the cutting edge on the punch works together with a counter cutting edge on the opening. During the following slotting process the strip also remains joined to the metal sheet on one side. To cut the strip free, the punch is rotated relative to the opening by 180°. Now the cutting edge on the punch already used at the beginning works together with the second counter cutting edge on the opening. The processing of the workpiece is performed with identical processing parameters for the initial cut stroke and the severing stroke.